Engine braking is used to control descent of a vehicle traveling along sloped terrain. Engine braking uses a compression stroke of the vehicle's internal combustion engine to dissipate energy transmitted to the engine from the vehicle's wheels through the vehicle's driveline and transmission. A vehicle driver may downshift the transmission into a lower transmission speed ratio and rely on engine braking to slow a descending vehicle. But on steep off-road terrain, engine braking alone may be insufficient, even in the lowest transmission speed ratio. Therefore, vehicle descent may be further controlled by automatically controlling the vehicle's brakes to supplement the engine braking.
For example, so-called hill descent control (HDC) systems maintain a constant low speed of a vehicle descending a steep hill without driver input. More specifically, HDC systems detect vehicle speed via wheel rotation, compare the detected vehicle speed to a target vehicle speed, and apply the vehicle's brakes when the detected vehicle speed exceeds the target vehicle speed. But this approach may not be optimal for all situations, such as when vehicle speed is difficult to assess because the vehicle's wheels are slipping, freewheeling, or momentarily locked up by the vehicle's brakes.